The present invention relates generally to a light emitting diode (LED), and more particularly, to an LED having a light transparent window layer by which the brightness of the LED can be enhanced with reduced manufacturing cost.
The LED has found wide application in diverse fields such as indicating lamps, display means, and sensors. In view of the efficiency of photon generation, most of LEDs are made of III-V compound semiconductor materials such as GaAs, InP, AlAs and their ternary or quaternary derivatives. For examples, an AlGaInP LED is formed, by successively growing of an n-type AlGaInP layer as a lower optical confinement layer, an AlGaInP layer as an active layer, and a p-type AlGaInP layer as an upper optical confinement layer, over an n-type GaAs substrate. The above LED is further provided with a front contact and a back contact on the top side and bottom side of the LED, respectively.
The above-mentioned upper optical confinement layer is generally of high electrical resistivity due to the requirement of higher band gap to keep it from absorbing light. However, the injected current is hard to spread out to the edge of the LED chip uniformly in case that the LED employs an upper optical confinement layer with high electrical resistivity. The radiation area of the active layer, therefore, is limited; and thus decreases the LED's radiation power and brightness. Moreover, since the confinement layer is not completely transparent, part of the light generated from the active layer are reflected back and absorbed by the absorbing substrate, rather than transmitting out. This process also limits the LED's brightness.
To enhance LED's brightness, a thick semiconductor layer as a transparent window layer has been proposed to cover the top of the LED, with reference to Table 1. However, the manufacturing processes for fabricating the above-mentioned transparent window layer are time consuming and of high cost. Moreover, other techniques involving the process of regrowth also have above drawbacks.
To prevent the problem of light absorption by opaque substrate, a reflecting mirror formed by a distributed Bragg reflector (DBR) is provided over the substrate. Nevertheless, the problem that injected current is hard to spread out to the edge of the LED chip uniformly is not solved satisfactorily in the prior arts. The uniformity of emitting light is accordingly degraded.